If it's an either/or situation, I would choose Rheum/ID over the derm elective because you'll learn from attendings that will give you a whole different experience and perspective than what you would get in derm (since you will be doing that residency anyway). Also, Rheum and ID are super-useful for derm.
i'd say do rheumatology. it's useful and once you start derm you'll be comanaging with rheum a lot. we send patients to rheum and have patients referred from rheum pretty much daily. you have the rest of your life to learn derm.
I don't think it really matters. Do whatever rotation is easiest. I don't remember most of the rheum i learned on my elective, except how to do a hand joint examination.
